Transistor Choice

Hi,

I'm having some difficulties finding the correct transistor. I'm designing a inductorless and transformerless RF power amplifier at

40Mhz, 1 Watt output. The design is somewhat different I guess. There is no impedance matching and I'm trying to maximise linearity and bandwidth. I'm operating on a 24V supply. I currently have a common- base stage, feeding into a shunt-feedback stage (gain) then onto a push-pull power amplifier.

I'm having trouble finding a relatively cheap option for the power stage. RF Power Transistors are generally too expensive and hard to obtain. Darlington are good but the poor Ft is a problem. Philips (NXP) have alot of good RF BJTs but the 24V supply really seems to limit my options. It seems I'm trying to find a transistor which isn't quite small-signal and isn't quite power, something in between. I've had a look at the older Motorola and NXP transistors and they would have been perfect, but these medium voltage, RF transistors seems to gone; replaced by small-signal RF.

Any help would be great.

Regards, Alan.

Reply to
jeeves666
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The reason you're struggling, Alan, is because this is a difficult problem. If you look at the CATV amplifier scene, you'll find that they've solved the problem, but only by using expensive, delicate transistors (mounted in expensive ceramic hybrid circuits), that are made in the fashion of complex integrated circuits. They have thousands of small low-voltage emitter-degenerated high-frequency transistors in parallel. It takes a high-magnification microscope to see what they've done. The result is a high-performance very wideband amplifier, but with fragile easy-to-burn-out properties.

I learned that running 24-volt amplifier modules at full power with 28V supplies (a 16% increase) was a quick path to disaster. They would enter an excessive-bias-current mode, and die. Only when I added a fast electronic fuse could I save them. Finally I lowered the voltage and breathed easier, having kept the electronic fuse.

Reply to
Winfield

The old 2n3375 had an Rb of 15 ohms, and was fairly robust. There was an RCA equiv., I forget the number, I'll try to find it today if you like.

Reply to
Jitt

Thanks Jitt, that would be great. I was wondering if there are many options for RF power transistors for push pull outputs ? Most I've seen are singular and don't have a complementary PNP device.

Reply to
jeeves666

Sorry for delay.... RCA had a series of those, 40279 and such, but I dont know where you'd get one today. As Win mentioned, these were for CATV work. I see the RCA 2n3375 shows Rb at 10 ohm, maybe the TI was 15- or maybe I'm wrong!

Reply to
Jitt

Some of those old RCA types ended up with a JEDEC number.

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Reply to
Michael A. Terrell

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